<p>This study presents a comprehensive investigation of the thermomechanical behavior of diamond reinforced metal matrix composites (DRMMCs) by developing a multiscale constitutive model without any empirical parameters. The model integrates effective inclusion theory with Mori-Tanaka homogenization and interfacial spring mechanic to accurately simulate the coupled response of DRMMCs simultaneously, including effective elastic response, average thermal expansion, and theoretical heat transport. By qualitatively revealing the underlying mechanism of thermomechanical response, the significant thermomechanical properties including the effective elastic constants (EECs), average coefficients of thermal expansion (CTE) and theoretical thermal conductivity (TC) of DRMMCs can be precisely predicted. Results indicate that the multiscale thermomechanical response involves the bidirectional interaction between temperature and stress fields across the multiscale architecture comprising diamond reinforcements, interfacial constituents, and metal matrix. This coupling is fundamentally driven by the thermomechanical mismatches in intrinsic properties between the constituents, while the interfacial constituents act as a critical regulatory mediator that regulate the intensity and direction of the coupling effect. Through the dynamic feedback loop of thermally induced stress-elastic deformation-heat transfer, the coupling effect directly determines the EECs, CTE, and TC of DRMMCs. This work provides a robust theoretical and modeling foundation for the design of high-performance thermal management materials with tailorable thermomechanical characteristics.</p>